Photoionization mass spectrometry has been used to measure the CH₂NH₂⁺ appearance energies for six primary amines, resulting in a 298 K heat of formation of 749.5 ± 1.3 kJ mol^–1 for the methylenimmonium cation. An absolute proton affinity of 868.8 ± 2.5 kJ mol^–1 is derived for methylenimine. Based on experimental data for ethylenediamine, the 298 K heat of formation for the aminomethyl radical is found to be 153.6 ± 1.7 kJ mol^–1. These results are supported by high-level ab initio calculations.

Keywords: methylenimmonium cation (CH₂NH₂⁺), methylenimine (CH₂NH), aminomethyl (CH₂NH₂), photoionization, proton affinity, heat of formation, appearance energy